This has been explained on Rokslide many times but here goes another…
For many loads (I know this works great for my .30-06, 6.5x55, 6CM, 6 ARC, .223) load for velocities given BC, altitude, scope height, typical temperature, etc such that you get the following elevation adjustment in Mils and this is super easy to do. Works great for loads at .400-.536 G1 BC from 2650-2800 MV.
100 yards =zero
300 yards = 1.0 mil
400 yards = 2.0 mil
500 yards = 3.0 mil
So if you range a target at 420 yards it takes no time at all to come up with 2.2 mils. For one of my loads this even works +/- 0.1 miles out to 700 yards